The present invention relates to mass spectrometers and more particularly, to a mass spectrometer suitable for measuring metastable ions.
Some of ions are so unstable thay they decompose near the exit of an ion source of the mass spectrometer in accordance with the following equation (1): EQU m.sub.o.sup.+ .fwdarw.m.sub.d.sup.+ +(m.sub.o -m.sub.d) (1).
The ion m.sub.o.sup.+ of a mass number of m.sub.o and the ion m.sub.d.sup.+ of a mass number of m.sub.d are usually called precursor ion and daughter ion, respectively.
Assuming now that, with a so-called double focusing mass spectrometer using electric and magnetic fields, the precursor ion m.sub.o.sup.+ is detected when the electric field is E.sub.o and the magnetic field is B.sub.o while the daugther ion m.sub.d.sup.+ is detected when the electric field is E.sub.d and the magnetic field is B.sub.d, the following equations stand as generally known in the art: EQU m.sub.d /m.sub.o =E.sub.d /E.sub.o ( 2) EQU m.sub.d.sup.2 /m.sub.o =m.sub.d.sup.* ( 3) EQU B.sub.d.sup.2 /B.sub.o.sup.2 =m.sub.d.sup.* /m.sub.o ( 4).
Accordingly, a combination of equation (3) with equation (4) is EQU m.sub.d /m.sub.o =B.sub.d /B.sub.o ( 5),
and a combination of equation (2) with equation (5) is EQU B.sub.o /E.sub.o =B.sub.d /E.sub.d ( 6).
These equations indicate that for the electric field being E.sub.d, the peak of the daughter ion m.sub.d.sup.30 having the mass number m.sub.d occupies a spectrum representative of a mass number m.sub.d.sup.* on the recorded spectra. The daughter ion m.sub.d.sup.+ is a very unstable ion resulting from the decomposition of the precursor ion near the exit of the ion source and is usually called metastable ion.
It is possible to measure the metastable ion through the use of a so-called linked scan type mass spectrometer in which the electric and magnetic fields are scanned at a constant ratio between the two fields so as to meet equation (6). In this measure, value of the electric field E.sub.o when detecting the precursor ion m.sub.o.sup.+ of the mass number m.sub.o is determined previously and a value of the electric field E.sub.d at which the metastable ion is detected is then determined. Thus, the mass number of the metastable ion m.sub.d.sup.+ calculated from equation (2). The value of the electric field E.sub.d is recorded on a recording paper for recording spectra or indicated on a voltmeter, and is determined by reading the recorded or indicated value. But the manual calculation is necessary for determination of the mass number of the metastable ion m.sub.d.sup.+, resulting in great inconvenience in data analysis.